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JOHNS HOPKINS UNIVERSITY MORPHOLOGICAL MONOGRAPHS. 



are shown migrating into the segmentation cavity under the edge of the 

 blastoderm, and filling the space between it and the yolk ; while in cut F 

 they are shown pushing in between the blastomeres, and some of them 

 are embedded in their substance. 



CUT E. 



CUT F. 



In Salpa there are two periods of migration. One, shown in Plate 

 XXXI, Figs. 5 and 6, which has never before been described, is very 

 early in the history of the ovarian eggs, when the migratory cells are 

 assimilated by the egg cell, and thus furnish the material for its growth. 

 A second period of migration, which was discovered and minutely 

 described by Salensky (5), begins immediately after fertilization, and 

 the migration goes on for a long time with great energy and rapidity 

 on an extensive scale, as shown in my Plates IX and X. 



Pyrosoma shows this migration of the follicle cells into the substance 

 of the embryo in a simpler form than Salpa, and while there may be no 

 conclusive evidence that the life-history of Salpa has come about by the 

 modification of that of Pyrosoma, there can be no doubt that the two 

 have had a common starting-point, and that, so far as the migration of 

 follicle cells goes, the Pyrosoma embryo is nearer this starting-point than 

 the Salpa embryo. It may therefore be used to interpret the structure 

 of the salpa embryo, and I shall attempt this without assuming that 

 the Pyrosoma embryo is in the direct line of modification. 



We have earlier stages in the evolution of the migratory follicle than 

 that exhibited by the Pyrosoma embryo. In cuts Gr and H, I have copied 

 from Davidoff two figures of early stages in the segmentation of Dis- 

 taplia magni larva. Cut Gr is an egg which has divided into two blasto- 

 meres, and cut H is one in which the ectoderm and endoderm are differ- 

 entiated. 



The egg is surrounded by a capsule of follicle cells, 6, which are 

 equivalent in ultimate origin to the germ cells, as they are in Salpa. In 

 Distaplia, according to Davidoff, the layer of cells by which the growing 

 egg cell becomes surrounded are so directly derived from cells which are 

 destined to become eggs, that this author calls them abortive eggs. 



